Bisphenols, and in particular bisphenol A (2,2 bis(p-hydroxyphenyl) propane, "BPA"), have become industrially significant reactants for a number of processes including the preparation of polycarbonates. Bisphenols are prepared on an industrial scale by one of two processes: an acid-catalyzed or HCI process and an ion exchange process, in which an acidic ion exchange resin such as sulfonic acid-substituted polystyrene is employed.
The bisphenol is prepared by condensation of two moles of phenol with one mole of a ketone or aldehyde, for example acetone, in the presence of an acidic catalyst. In addition to the bisphenol, however, the product stream from the reaction includes unreacted phenol, which is included in excess over the stoichiometric requirement, and various isomers of the desired bisphenol and other by-products. Because these by-products can compromise the properties of products made using the bisphenol, they need to be separated. One technique for accomplishing this separation involves cooling the product stream to induce crystallization of a 1:1 bisphenol:phenol adduct which can then be further processed by washing, distillation, extraction and/or steam stripping to produce a purified bisphenol product. Processes for the production and purification of bisphenols are well known, and are described inter alia in U.S. Pat. Nos. 4,107,218; 4,294,994; 5,210,329; 5,243,093; 5,245,088; 5,288,926; 5,368,827; 5,786,522; and 5,874,644.
In some semicontinuous procedures for the preparation of bisphenols, phenol and acetone are mixed and partially reacted in a pre-reactor. The partial reaction is allowed to proceed through 10 to 70% conversion, and the resulting product is then transferred to a batch reaction in which full conversion of the starting materials is achieved. The temperature in the pre-reactor is selected so that a little, BPA is precipitated. These crystals serve as seed crystals in the batch reactor. However, crystallization of BPA tends also to form a layer on interior surface of the pre-reactor, particularly the cooling surfaces which are used to maintain the temperature in the pre-reactor. This layer hinders heat transfer between these cooling surfaces and the reaction mixture. This reduced heat transfer can cause lower production rates and/or reduced purity of the finished product and is therefore undesirable.
To prevent these side effects, it is conventional to clean the pre-reactor periodically. This is done by either filling the pure phenol or with a normal charge where extra water or acid is added. These procedures increases the solubility of BPA and the removal of the BPA layer from the cooling surfaces, but the product produced during these cleaning procedures does not meet product specification. Thus, the time spent cleaning is wasted from a productivity point-of-view, and there is a need for an improved method for manufacture of bisphenols, in which this disruption is reduced or eliminated.